Lighter-than-air aircraft and related methods for powering the same

ABSTRACT

A lighter-than-air aircraft includes a gas envelope for containing a buoyant gas, and a solar panel is carried by a predetermined portion of the gas envelope. A solar sensor is used for determining a direction of the sun. A propulsion system carried by the gas envelope orients the gas envelope so that the solar panel is oriented in the direction of the sun based upon the solar sensor. A navigation controller cooperates with the propulsion system to move the lighter-than-air aircraft along a desired flight path while the solar panel is oriented in the direction of the sun.

FIELD OF THE INVENTION

The present invention relates to the field of lighter-than-air aircraft,and in particular, to a lighter-than-air aircraft capable of remainingin the air at high altitudes for extended periods of time.

BACKGROUND OF THE INVENTION

High altitude, long-duration solar powered aircraft have been proposedfor both commercial and military applications. For example,lighter-than-air aircraft have been proposed for cellular telephoneapplications. Military applications also include telecommunicationapplications as well as providing surveillance.

There is a domain in the upper stratosphere at 60,000 feet where it isideal to position a lighter-than-air aircraft. This altitude allowson-board sensors to see over the horizon at least 350 miles in anydirection. In most such applications, long duration station keeping isessential. Consequently, the issue is not in getting an aircraft to60,000 feet, but in maintaining power so that the on-board sensors andelectronics are continuously powered for extended periods of time, whichmay be from a few weeks to several months to even longer.

Electrical energy generated using solar cells or photovoltaic cells aretypically used to power lighter-than-air aircraft. For example, U.S.Patent Application No. 2002/0005457 discloses a lighter-than-airaircraft powered with flexible solar cells integrated within thematerial covering the aircraft. Although the energy provided by solarcells is adequate to power lighter-than-air aircraft while in thesunlight, the challenge is to repeatedly get through the night. To keepa large lighter-than-air aircraft in a general location at 60,000 feetrequires a significant amount of power. The solar panels not only needto take in enough solar energy to power the aircraft during the day, butalso needs to take in additional power to be stored in batteries so thatit can be used during the night.

In addition, extra power is needed to maintain position due to the upperwinds or air currents at 60,000 feet, and for maintaining direction ofthe solar panels toward the sun as the direction of the sun changesthroughout the day. This puts a bigger demand on the ability to storepower for use during the night. One approach is to place more solarpanels on the aircraft for collecting and storing the additional power,but this results in an increase of the weight of the aircraft. Thegreater the weight, the greater the volume of lift gas required, whichincreases the amount of material necessary to contain the lift gas.These increases in weight and volume impose additional powerrequirements.

As an alternative to placing more solar panels on the aircraft, oneapproach is to maintain an optimum position of the solar cells inrelationship to the sun. For example, most all spacecraft are solarpowered. In such spacecraft, the solar panels are rotatable so that anoptimum angle can be maintained between the solar panels and the sun.However, these systems are not particularly advantageous on alighter-than-air aircraft. In U.S. Pat. No. 6,371,409, solar panelsmounted on an outer surface of a lighter-than-air aircraft are movableover a portion of the surface thereof to adjust for changes in thedirection of the sun, or if maintained in a stationary position, for theinclination of the sun throughout the day.

Another approach to providing the power needed throughout the night isto use fuel cells. For example, the power requirements for the highaltitude airship (HAA) as designed by Lockheed Martin Corp. are met by acombination of solar cells, fuel cells and batteries, wherein the fuelcells provide electrical power during the night. The fuel cells receivethe gaseous elements of hydrogen and oxygen for generating electricalpower.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to increase the efficiency at which energy iscollected, stored and converted to power so that a lighter-than-airaircraft can remain aloft at high altitudes for extended periods of timewithout having to return to ground for refueling.

This and other objects, features, and advantages in accordance with thepresent invention are provided by a lighter-than-air aircraft comprisinga gas envelope for containing a buoyant gas, at least one solar panelcarried by a predetermined portion of the gas envelope, and at least onesolar sensor for determining a direction of the sun.

A propulsion system carried by the gas envelope orients the gas envelopeso that the solar panel is oriented in the direction of the sun basedupon the solar sensor. The lighter-than-air aircraft may furthercomprise a navigation controller cooperating with the propulsion systemto move the lighter-than-air aircraft along a desired flight path whilethe solar panel is oriented in the direction of the sun. Pointing thesolar panel in the direction of the sun independent of the desiredflight path increases the efficiency at which solar energy is collected.

The lighter-than-air aircraft is capable of high-altitude stationkeeping within altitude and perimeter boundaries for extended periods oftime. The lighter-than-air aircraft is intended to operate at analtitude of about 60,000 to 80,000 feet in the stratosphere, where it isideal to sit, look, listen and provide surveillance and communicationsfrom a strategic perspective. This altitude allows on-board sensors tosee over the horizon at least 350 miles in any direction.

Since the propulsion system orients the gas envelope so that the solarpanel is oriented in the direction of the sun based upon the solarsensor, this allows the solar panel to be constantly pointing toward thesun. With the performance of the solar panel being optimized, extrasolar panels do not need to be carried by the gas envelope. Thisadvantageously reduces the overall weight and cost of thelighter-than-air aircraft.

The gas envelope may be substantially symmetrical about a vertical axisand may comprise an upper portion having a partial spheroidal shape. Thesolar panel may be carried by a predetermined segment of the partialspheroid. As a result of the symmetry of the gas envelope, the solarpanel may be placed on any side thereof and still be optimized forcollecting solar energy when facing the direction of the sun. Incontrast, the direct front or rear of a traditional blimp has little orno solar exposure since the blimp does not have symmetry about itsvertical axis.

The propulsion system may comprise an electrical propulsion system. Thelighter-than-air aircraft may further comprise a closed loop fuel cellcarried by the gas envelope for powering the electrical propulsionsystem when the solar panel is not generating sufficient electricity,and having its fuel regenerated by the solar panel from its exhaust whenthe solar panel is generating sufficient electricity. The closed loopfuel cell advantageously increases the efficiency at which fuel isstored and converted to power so that the lighter-than-air aircraft canremain aloft at high altitudes for extended periods of time withouthaving to return to ground for refueling.

The lighter-than-air aircraft may further comprise a support structurewithin the gas envelope. The support structure moves the gas envelopefrom a retracted position to an expanded position as the buoyant gasexpands due to an increase in altitude of the lighter-than-air aircraft.A gondola may also be carried by the gas envelope, and as the gasenvelope moves from the retracted position to the expanded position, thegondola may be lowered from the gas envelope as the later expands.

In the retracted position, the gas envelope and gondola have less dragbecause of its “flat top,” and because the gondola is pulled closer tothe gas envelope. This results in the gas envelope and gondola having areduced cross section, which helps to reduce the effects of winds at thelower altitudes. Another advantage of the “flat top” design is that itallows for a significant reduction in the height of the facilityconstructing the lighter-than-air aircraft. Once thelighter-than-aircraft reaches its desired altitude near or above 60,000feet, the gas envelope is fully expanded.

The propulsion system may comprise a plurality of spaced apartpropellers extending outwardly from the gondola. A respective electricmotor may drive each of the propellers. The propulsion system mayfurther comprise a respective gimbal coupled to each of the propellers.

Another aspect of the present invention is directed to a method foroperating a lighter-than-air aircraft comprising a gas envelope forcontaining a buoyant gas, at least one solar panel carried by apredetermined portion of the gas envelope, at least one solar sensor,and a propulsion system carried by the gas envelope. The methodcomprises determining a direction of the sun based upon the at least onesolar sensor, and using the propulsion system for orienting the gasenvelope so that the at least one solar panel is oriented in thedirection of the sun.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lighter-than-air aircraft at highaltitude providing surveillance and communications about a desiredlocation on earth in accordance with the present invention.

FIG. 2 is an enlarged perspective view of the underside of thelighter-than-air aircraft as shown in FIG. 1 illustrating in greaterdetail the gondola and fuel storage holders.

FIG. 3 is an enlarged view of the gondola as shown in FIG. 2illustrating in greater detail the propulsion system for thelighter-than-air aircraft.

FIGS. 4 a-4 e illustrate various positions of the propulsion systemresulting in a navigation vector that varies while the solar panel iscontinuously pointed in the direction of the sun for thelighter-than-air aircraft in accordance with the present invention.

FIGS. 5 a-5 f illustrate various positions of the propulsion systemresulting in a navigation vector that remains constant while theposition of the solar panel varies for tracking the sun during the dayfor the lighter-than-air aircraft in accordance with the presentinvention.

FIG. 6 is a cross-sectional side view of the lighter-than-air aircraftillustrating the support structure within the gas envelope wherein theupper portion of the gas envelope is in a retracted position inaccordance with the present invention.

FIG. 7 is a cross-sectional side view of the lighter-than-air aircraftillustrating the support structure within the gas envelope wherein theupper portion of the gas envelope is in an expanded position inaccordance with the present invention.

FIGS. 8 a-8 c are perspective views of the gas envelope changing from aretracted position to an expanded position as the altitude of thelighter-than-air aircraft increases in accordance with the presentinvention.

FIG. 9 is a perspective view of the gas envelope illustrating variousangles of solar incidence for the solar panel in accordance with thepresent invention.

FIG. 10 is a block diagram of a closed loop combustion generator forgenerating electricity for the lighter-than-air aircraft in accordancewith the present invention.

FIG. 11 is a block diagram of another embodiment of the closed loopcombustion generator as shown in FIG. 11.

FIG. 12 is a block diagram of a closed loop fuel cell for generatingelectricity for the lighter-than-air aircraft in accordance with thepresent invention.

FIG. 13 is a block diagram illustrating the on-board electronics carriedby the lighter-than-air aircraft in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout, and prime, and double primenotations are used to indicate similar elements in alternativeembodiments.

Referring initially to FIGS. 1-3, the lighter-than-air aircraft 20 iscapable of high-altitude station keeping within altitude and perimeterboundaries for extended periods of time. The illustratedlighter-than-air aircraft 20 is in the upper stratosphere at 60,000 to80,000 feet, for example, where it is ideal to sit, look, listen andprovide surveillance and communications from a strategic perspective.This altitude allows on-board sensors to see at least 350 miles in anydirection. For example, the lighter-than-air aircraft 20 may providesurveillance 21 about a location of interest 23 on the surface of theearth, and provide this information to a command and control center 25via a communications link 27, as illustrated in FIG. 1. Thelighter-than-air aircraft 20 may also provide over the horizonsurveillance or communications 29.

The lighter-than-air aircraft 20 is unmanned, and communications 27 isprovided with the ground based command and control center 25.Communications 27 may be directly between the command and control center25 and the lighter-than-air aircraft 20. Alternatively, communications27, 29 may be provided via a relay satellite or other airborne platform.

The lighter-than-air aircraft 20 comprises a gas envelope 22 containinga buoyant gas, and at least one solar panel 24 is carried by apredetermined portion of the gas envelope. The at least one solar panelmay be one large solar panel, or may be a solar array comprising aplurality of smaller solar panels. For purposes of discussion, the atleast one solar panel 24 will simply be referred to as the solar panel.The solar panel 24 may be integrated into the skin of the gas envelope22, or may be separate from the skin, as readily appreciated by thoseskilled in the art. The buoyant gas, for example, may be helium,hydrogen or combinations thereof, or other combinations oflighter-than-air gasses.

At least one solar sensor 26 determines a direction of the sun basedupon the incident light rays received from the sun. The solar sensor 26may be separate from the solar panel 24, as illustrated. Alternatively,the solar sensor 26 may be integrated within the solar panel 24 fordetermining the direction of the sun based upon the incident light rays.The illustrated solar sensor 26 is located on top of the gas envelope22. In another embodiment, a plurality of solar sensors 24 are spacedaround the gas envelope 22.

A propulsion system 28 orients the gas envelope 22 so that the solarpanel 24 is oriented in the direction of the sun based upon the solarsensor 26. This advantageously allows the solar panel 24 to beconstantly pointing toward the sun. Since the performance of the solarpanel 24 is optimized, extra solar panels do not need to be carried bythe gas envelope 22, which reduces the overall weight, complexity andcost of the lighter-than-air aircraft 20.

A gondola 30 is carried by the gas envelope 22. As will be discussed ingreater detail below, power conversion, management functions and thepropulsion system 28 are an integral part of the gondola 30. Many ofthese items are carried by the payload bay 31 of the gondola 30. Inparticular, the payload bay 31 carries the electronics, communicationsand/or surveillance equipment. Fuel storage is above the gondola 30 andis enclosed by the gas envelope 22. The fuel storage includes hydrogenand oxygen fuel holders 32, 34 for respectively storing the gaseouselements of hydrogen and oxygen to be used for powering the propulsionsystem 28. The water that is broken down into hydrogen and oxygen gasesis carried in the gondola 30.

The propulsion system 28 comprises a plurality of spaced apartpropellers 40 extending from the gondola 30. Each propeller 40 can beindependently driven, or the propellers can all be driven together. Inthe illustrated embodiment of the propulsion system 28, six booms 42 areattached to the gondola 30 for supporting six independent drives 44,i.e., six electric motors. Each boom 42 thus supports a respectiveelectric motor 44 for driving the propeller 40 coupled thereto. Theactual number of motors/propellers can vary depending on their size andthe size of the lighter-than-air aircraft 20.

Each electric motor 44 is also coupled to a dual axis gimbal 46. Thedual axis gimbals 46 advantageously allow the propellers 40 to bepositioned so that the lighter-than-air aircraft 20 can move in anydirection, similar to a helicopter. An advantage of the propulsionsystem 28 is that the lighter-than-air aircraft 20 can move in anydirection while the solar panel 24 is continuously being pointed in thedirection of the sun. In other words, the navigation vector of thelighter-than-air aircraft 20 can vary while the sun vector associatedwith the angle of the solar panel 24 pointed in the direction of the sunremains constant toward the sun.

An example of the solar panel 24 being continuously pointed toward thesun while the navigation vector changes is illustrated in FIGS. 4 a-4 e.The navigation vector 50 represents the direction and motion of thelighter-than-air aircraft 20. Even if the lighter-than-air aircraft 20is not moving, the navigation vector 50 may vary to compensate for winddirection and speed. In FIG. 4 a, the propellers 40 are rotated so thatthe navigation vector 50 is at −30 degrees while the sun vector 52 is at90 degrees. The sun vector 52 represents the direction the solar panel24 is pointing.

If the navigation vector 50 changes to 30 degree, the propellers 40carried by the gondola 30 are rotated accordingly while the sun vector52 remains constant at 90 degrees, as illustrated in FIG. 4 b. The sameconcept applies when the navigation vector 50 changes to 20, 10 and 0degrees, as illustrated in FIGS. 4 c, 4 d and 4 e.

An example of the navigation vector 50 being constant while the sunvector 52 changes is illustrated in FIGS. 5 a-5 f. With thelighter-than-air aircraft 20 holding a fixed position, the gas envelop22 needs to rotate as the sun rises and sets during the day so that thesolar panel 24 remains constantly pointed toward the direction of thesun.

At 8 am, for example, the sun vector 52 is at 10 degrees, as illustratedin FIG. 5 a. At 10 am, the sun vector 52 is at 45 degrees, but thisrequires the propellers 40 that are carried by the gondola 30 to berotated so that the solar panel 24 follows the direction of the sunwhile the navigation vector 50 remains constant, as illustrated in FIG.5 b. The process is repeated throughout the day as the sun changesposition, as illustrated in FIGS. 5 c-5 f.

In the illustrated lighter-than-air aircraft 20, the gas envelope 22 andthe gondola 30 are fixed. That is, when the gas envelope 22 rotates, sodoes the gondola 30. This embodiment requires the motors 44 to operatein a sequence with a stepwise re-clocking of the propellers 40 when theyhave been rotated as far as they can rotate for maintaining a constantpointing of the solar panel 24 toward the direction of the sun. Forexample, when a first motor in the sequence of motors reaches itsmaximum allowable gimbal rotation, it simply slows and rotatesapproximately 180 degrees and becomes the last motor in the sequence ofmotors. The sequence of the motors continues to change as necessarybased upon the desired navigation and/or solar vector. Also, the thrustdirection of each re-clocked propeller 40 is reversed.

In another embodiment, the gas envelope 22 and the gondola 30 rotateindependently from one another, much like a turret on a tank. Thegondola 30 may rotate as necessary to maintain a desired flight pathvector while the solar panel 24 remains in the direction of the sun.

Referring now to FIGS. 6 and 7, the gas envelope 22 comprises a supportstructure for moving THE gas envelope from a retracted position (FIG. 6)to an expanded position (FIG. 7). The support structure comprises ahoop-truss member 60 having a ring shape. The hoop-truss member 60 isderived from hoop antennas that are deployed in space, as readilyappreciated by those skilled in the art. The hoop-truss member 60includes a number of compressive members and stabilizing tension cords62 for providing the necessary support. Other internal design structuresare acceptable as readily appreciate by those skilled in the art, suchas a radial rib structure, for example.

The gondola 30 is attached to the hoop-truss member 60 via attachments64, and to a control member 66 via attachments 71. The control member 66is above the hoop-truss member 60. Fuel storage holders for theapplicable gases are above the gondola 30, and are enclosed by the gasenvelope 22. The fuel storage holders as noted above include hydrogenand oxygen fuel holders 32, 34 for respectively storing the gaseouselements of hydrogen and oxygen to be used for powering the propulsionsystem 28 during the night.

The control member 66 enables volumetric control of the upper portion ofthe gas envelope 22 during ascent and decent. As the buoyant gas expandsor contracts as a function of the altitude, the volume of the gasenvelope 22 changes accordingly. Although not shown in the figures, aperimeter stabilized inflatable structure, in concert with the morestable rigid members 60 and 66, may also be used to provide support ofthe desired contour of the gas envelope 22. An approach of using radialmembers within the cord structure allows the creation of a substantiallycircular shape.

Volumetric control of the gas envelope 22 may be performed manually orautomatically. Small electric motors 68 are positioned around thecontrol member 66, and retract or release tie-downs 70 attached to theupper surface of the gas envelope 22, and tie-downs 71 attached to thegondola 30. The electric motors 68 are not limited to being locatedaround the control member 66. They may be located around the hoop-trussmember 60, for example. The gondola 30 carries an altimeter 72 fordetermining the altitude of the lighter-than-air aircraft 20, andprovides the altitude to an envelope controller 74 or measurement ofbarometric pressure/relative pressure.

The altimeter 72 and the controller 74, as well as other on-boardelectronics and sensors, will be discussed in greater detail whenreference is made to FIG. 13. The envelope controller 74 operates thesmall electric motors 68 so that the tension cords or tie-downs 70, 71are either retracted or released based upon the altitude. This featureof the present invention advantageously allows for the expansion of thebuoyant gas as the lighter-than-air aircraft 20 traverses the atmosphereto the desired station keeping altitude.

The desired altitude of the lighter-than-air aircraft 20 is preferablyin the stratosphere, which corresponds to an altitude of 60,000 feet orhigher. Of course, the lighter-than-air aircraft 20 may operate at loweraltitudes depending on its intended purpose.

When the lighter-than-air aircraft 20 is in the lower atmosphere, theupper portion of the gas envelope 24 is retracted toward the controlmember 66, and the gondola 30 is also retracted toward the controlmember as illustrated in FIG. 6. This reduces the cross-sectional areaof the gas envelope 24, which results in a low profile, i.e., a reduceddrag. The winds in the denser air of the lower atmosphere have asignificant effect on large structures, such as the lighter-than-airaircraft 20.

When the gas envelope 22 is fully collapsed, the height h₁ of theillustrated gas envelope is 80 feet, and the height h₂ including thegondola 30 is 97 feet. When the gas envelope 22 is fully expanded, asillustrated in FIG. 7, these dimensions h₁, h₂ are respectively 96 feet,148 feet. The width w₁ of the gondola 30 is 22 feet, and the overallwidth w₂ of the lighter-than-air aircraft 20 is 215 feet. The height h₃of the hoop-truss member 60 is 24 feet, and the height h₄ between thehoop-truss member and the top of the gas envelope 22 is 91 feet. Theradius r₁ of the upper portion of the gas envelope 22 when fullyexpanded is 118 feet, whereas the radius r₂ of the lower portion of thegas envelope is 272 feet. The inside diameter of the hoop-truss member60 is 161 feet. These numbers are for illustrative purposes only, andthe actual size of the lighter-than-air aircraft will vary depending onthe intended application.

FIGS. 8 a-8 c are perspective views of the gas envelope 22 changing fromthe retracted position to the expanded position as the altitude of thelighter-than-air aircraft 20 increases. In the retracted position, thegas envelope 22 has a low drag because of its “flat top” and because thegondola 30 is pulled or held closer position toward the gas envelope, asshown in FIG. 8 a. Because of the reduced cross section, this helps toreduce the effects of winds at the lower altitudes. As thelighter-than-air aircraft 20 increases in altitude, the buoyant gasexpands so that the volume of the gas envelope 22 increases and thegondola 30 is lowered away from the gas envelope, as shown in FIG. 8 b.Once the lighter-than-aircraft 20 reaches its desired altitude near orabove 60,000 feet, the gas envelope 22 is fully expanded and the gondola30 is in its resting position, as shown in FIG. 8 c.

Another advantage of the “flat top” design is that it allows for asignificant reduction in the height of the facility constructing thelighter-than-air aircraft 20. The lighter-than-air aircraft 20 may beconstructed at the reduced height, and then moved outside fordeployment.

The flexible material covering the hoop-truss member 60 and the controlmember 66 is preferably a high strength material. This material may bemade from Kapton films, Tedlar, and Vectran, for example. The materialmay also comprise a polyester film, and may also be a combination ofdifferent materials. For example, Vectran may be used for the loadbearing fabric. Tedlar and polyester film laminates may form theultraviolet protection layer, and also function as a gas barrier. Thesematerials have a high resistance to radiation and to cold temperatures.

An advantage of the present invention is that the gas envelope 22 may beconstantly pointed in the direction of the sun. The gas envelope 22 issubstantially symmetrical about its vertical axis and comprises an upperportion having a partial spheroidal shape. This shape advantageouslyprovides for good solar incidence 360 degrees around the perimeter ofthe gas envelope 22, and at low elevation angles.

The solar panel 24 is carried by a predetermined angular segment of thepartial spheroid. Out of a total angular segment of 360 degrees, thepredetermined angular segment is within a range of about 60 to 120degrees, for example, with about 90 degrees being illustrated in thefigures. In contrast, the direct front or rear of a blimp has little orno solar exposure due to its lack of symmetry about a vertical axis. Asa result of the spheroidal shape of the gas envelope 22, the solar panel24 may be placed on any side thereof and still be optimized forcollecting solar energy via the solar panel facing the direction of thesun. Since the effectiveness of the solar panel 24 is directly relatedto the incidence angle of the sunlight, it becomes very important tooptimize these pointing angles.

Various example positions of the sun above the horizon and its footprinton the solar panel 24 are shown in FIG. 9. For example, reference 53represents the sun 0° above the horizon with a +/−40° view angle,reference 54 represents the sun 14° above the horizon with a +/−40° viewangle, reference 55 represents the sun 28° above the horizon with a+/−35° view angle, reference 56 represents the sun 42° above the horizonwith a +/−30° view angle, and reference 57 represents the sun 56° abovethe horizon with a +/−25° view angle. In addition, the solar panel 24 isplumbed back to the gondola 30 using reinforced channels within thesolar surface and routing through portions of the inner supportstructure. The solar panel 24 thus has an efficient overall incidentarea when directed toward the sun. As a result of the additional weightof the solar panel 24 on one side of the gas envelope 22, the gondola 30should be slightly off center or internal elements should be adjusted tobalance the center of gravity.

As an alternative embodiment resulting from the gas envelope 22 beingsymmetrical about its vertical axis, solar panels 24 may be placed allthe way around so that it does not matter which direction the gasenvelope is pointing. Consequently, the use of the solar sensor 26 is nolonger necessary. This embodiment may be particularly attractive if thetechnology for solar panels allows for light weight solar panels, andthe impact of placing them all the way around the gas envelope 22 is nottoo detrimental to the overall weight and performance of thelighter-than-air aircraft 20.

Referring now to FIGS. 10-12, various embodiments for generatingelectricity for the lighter-than-air aircraft 20 during the night cyclewill now be discussed. It is worth noting that these differentembodiments for generating electricity may also be used on other typesof aircraft, including those that are heavier-than-air, as readilyappreciated by those skilled in the art.

The sun is generally available for about 8 hours during the day in whichextra electricity is generated beyond what is required for powering thelighter-than-air aircraft 20. Availability of the sun is highlydependent on location of the lighter-than-air aircraft 20 relative tothe equator and on the time of the year. This extra electricity is usedfor regenerating fuel, which is then used for generating electricityduring the night cycle. There are an additional 1.5 hours in the morningand 1.5 hours in the evening where the sun provides enough solar energyfor powering the lighter-than-air aircraft 20, but does not generate anyextra electricity. The night cycle is about 13 hours where there iseffectively no sunlight available.

In the illustrated embodiment of the lighter-than-air aircraft 20, it isestimated that about 750 W-hr/kg is required. However, current batterytechnology offers about 150 W-hr/kg storage potential. Consequently,these batteries are not efficient enough, per unit of weight, for themto be a good choice for powering the lighter-than-air aircraft 20 duringthe night.

In one embodiment, a closed loop combustion generating system 80 powersthe propulsion system 28 when the solar panel 24 is not generatingsufficient electricity (i.e., during the night), and has its fuelregenerated by the solar panel from its exhaust when the solar panel isgenerating sufficient electricity (i.e., during the day). The closedloop combustion generating system 80 comprises a combustion generator 82for receiving the fuel, and for generating a pressurized gas based uponcombustion of the fuel. The combustion generator 82 may comprise aturbine generator or a piston generator, for example, for generatingelectricity and producing exhaust 90 as a result thereof.

The closed loop combustion generating system 80 comprises a condenser 88for condensing the exhaust 90 from the combustion generator 82 to aliquid. The condenser 88 takes advantage of the cold ambient night toremove heat from the exhaust. In the illustrated embodiment, thecondenser 88 is carried by the gas envelope 22 adjacent the solar panel24. The condenser 88 is spread out adjacent the solar panel 24, whichacts as a radiator for removing heat, i.e., a large heat sink potential.With the ambient air being about −70° F. at 60,000 feet, and the heatsink potential being about 18 W/ft², the solar panel 24 can effectivelyfunction as a radiator. In another embodiment, the condenser 88 iscarried by the gondola 30, and air may be forced over the condenser tohelp condense the exhaust 90 to a liquid.

At least one converter 86 converts the liquid from the condenser 88 backinto fuel when electricity is being input from the solar cell 24, i.e.,during the day. The fuel comprises hydrogen gas and oxygen gas so thatthe exhaust comprises water. The converter 86 comprises an electrolyzerfor breaking the water down during the day into the hydrogen and oxygengases, which are stored in respective fuel storage holders 32, 34. Thisfuel is then used during the night cycle for generating electricity. Thewater is stored in a water storage holder 78 in the gondola 30.Insulation and mini-heaters are used to keep the water from freezing atthe high operating altitudes of the lighter-than-air aircraft 20.

If the water ever needs to be replenished while the lighter-than-airaircraft 20 is in flight, the aircraft may drop its altitude so that itis in the clouds. Once the lighter-than-air aircraft 20 is in theclouds, water may be collected, as readily appreciated by those skilledin the art. Along these same lines, if the buoyant gas in the gasenvelope 22 needs to be replenished, then a portion of the hydrogen gasin the hydrogen gas storage holder 32 may be added to the gas envelope.

A fuel cell 110 may also be used for combining the hydrogen and oxygengases for generating electricity. A by-product 112 of combining thehydrogen and oxygen gases in the fuel cell 110 is water 112, which isrouted to the water storage holder 78.

The closed loop combustion generating system 80 may also includes asecond generator 100 for generating electricity. A portion of the water102 from the condenser 88 or a portion of the water 112 from the fuelcell 110 may be routed to the combustion generator 82. The combustiongenerator 82 can reach temperatures of about 5800° F., and the heatgenerated by the combustion chamber is used to heat the water.

Once the water is heated to a pressurized gas, it is applied to thesecond generator 100. The pressurized gas may drive a turbine, asillustrated, or a piston, for example, for generating electricity. Theexhaust 104 exiting the second generator 100 is then combined with thehydrogen and oxygen gases within the combustion generator 82.Effectively, this is a reheat stage that includes the addition of thenew combustion gas products.

In another embodiment, the closed loop combustion generator 80′ is basedupon the use of a vaporization fluid such as butane or propane forgenerating electricity, as shown in FIG. 11. The elements having thesame reference numerals as in FIG. 10 perform the same function and willnot be discussed.

Liquid butane or propane 130′ is first routed from a supplemental fuelholder 132′ to the fuel cell 110′. The fuel cell 110′ is about 50%efficient, which means the heat generated by the fuel cell whengenerating electricity may be used for heating the butane or propane.The butane or propane will also be referred to as a supplemental liquid130′.

In lieu of propane or butane, another liquid or gas having similarproperties may be used as the supplemental liquid. These propertiesinclude low vapor pressure at temperatures between −30° F. and −70° F.,and a much higher vapor pressure at temperatures between 110° F. and180° F. For example, the supplemental liquid has gas properties of 0psig vapor pressure at −60° F. (in the condenser 140′), and between150-200 psig at 110° F. (at the fuel cell 110′). Propane or butane, forexample, condenses to a liquid at about −60° F., which is the sametemperature as the ambient atmosphere at 60,000 feet. Thermal removalrate is about 18 W/ft².

The heat generated by the fuel cell 110′ is used to pre-heat thesupplemental liquid 130′. When the supplemental liquid 130′ is heated,it vaporizes at a much lower temperature. As it heats, the liquid butaneor propane turns into a gas. The goal is to convert from liquid to vaporwithin the fuel cell 110′ which maximizes the effective heat transferassociated with the latent heat of vaporization. The gas is routed tothe combustion generator 82′. As the gas is heated even higher, itbecomes more unstable and becomes a pressurized gas which increases thevolume that is maintained near constant pressure.

The pressurized gas 103′ is used to drive a second generator 100′ forgenerating electricity. The exhaust 104′ from the second generator 100′is routed to a second condenser 140′. The condensed supplemental exhaust105′ is routed to the supplemental liquid holder 132′. In theillustrated embodiment, the second condenser 140′ is also carried by thegas envelope 22′ adjacent the solar panel 24′. In another embodiment,the second condenser 140′ is carried by the gondola 30′, and air may beforced over the condenser to help condense the gas to liquid form. Theexhaust 104′ will be in the form of an expanded gas. The ambienttemperature will cool the gas back to the supplemental liquid 130′ (apoint of re-liquefaction/condensing). This process for the supplementalliquid 130′ does not occur naturally at or near the earth's surface, forinstance, below 20,000 feet altitude.

In yet another embodiment of generating electricity during the night, aclosed loop fuel cell 80″ is used, and the supplemental liquid 130″ isheated by the fuel cell 110″. The supplemental liquid 130″ is heateduntil it becomes a pressurized gas 133″. The pressurized gas 133″ isused to drive a generator 84″ for generating electricity. The generator84″ is a turbine generator or a piston generator, for example.

The exhaust 90″ from the generator 84″ is routed to a condenser 88″. Thecondensed supplemental exhaust 136″ is then routed to the supplementalliquid holder 132″. As in the previous embodiments, the condenser 88″ isalso carried by the gas envelope 22″ adjacent the solar panel 24″ sothat it operates as a heat sink during the night. In another embodiment,the condenser 88″ is carried by the gondola 30″, and air may be forcedover the condenser to help condense the gas to liquid form.

Another advantage of this particular embodiment is that the system canbe reversed during the day for generating electricity. That is, thesupplemental liquid is heated by the solar panel 24 so that it becomes apressurized gas for driving a generator for generating electricity, asreadily appreciated by those skilled in the art. Further, thesupplemental liquid is re-condensed in the gondola 30 by ambient airforced over a heat exchanger, as readily appreciated by those skilled inthe art.

The on-board electronics carried by the lighter-than-air aircraft 20will now be discussed with reference to FIG. 13. The avionics 150required to support the lighter-than-air aircraft includes a number ofdifferent type communications links. A first communications link 152 isa two-way, line-of-sight system capable of uploading commands forcontrolling the aircraft's 20 systems and payloads, and downloading thestatus of all on-board systems and mission payload data. Thiscommunications link may operate at the Ku-band and is capable ofproviding uplink rates of at least 200 kbps and downlink rates from 2Mbps to 274 Mbps.

A second communications link 154 includes one or more satellitecommunication systems to be used for both vehicle and payload controland monitoring as well as transmission of payload data. A thirdcommunications link 156 includes VHF/UHF radios for providing a directcommunications path to air traffic controllers. It also allows aremotely located “pilot” to communicate with a controller, thusproviding a standard interface to the world. The avionics 150 alsoincludes a radar 158 and a camera 159.

The navigation controller 160 cooperates with the propulsion system 28to move the lighter-than-air aircraft 20 along a desired flight pathwhile the solar panel 24 is oriented in the direction of the sun. Thenavigation controller 160 receives information on the location of thelighter-than-air aircraft 20 from a GPS receiver 162. An altimeter 170provides altitude information to an envelope controller 172 forcontrolling the profile of the gas envelope 22 based upon the altitude.As discussed above, the gas envelope 22 may be in a retracted positionat low altitudes, but as the altitude increases and the buoyant gasexpands within the gas envelope, then the envelope controller 172 placesthe gas envelope in the expanded position.

Flight controls/mission computer 180 interfaces with the otherelectronic devices on-board for providing overall control of thelighter-than-air aircraft 20. An aircraft condition analysis andmanagement system (ACAMS) 182 is also carried by the lighter-than-airaircraft 20 for providing aircraft diagnostics.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings. Inaddition, other features relating to the lighter-than-air aircraft isdisclosed in the copending patent application filed concurrentlyherewith and assigned to the assignee of the present invention and isentitled LIGHTER-THAN-AIR AIRCRAFT INCLUDING A CLOSED LOOP COMBUSTIONGENERATING SYSTEM AND RELATED METHODS FOR POWERING THE SAME, ApplicationSer. No. 10/977,791, the entire the entire disclosure of which isincorporated herein in its entirety by reference. Therefore, it isunderstood that the invention is not to be limited to the specificembodiments disclosed, and that modifications and embodiments areintended to be included within the scope of the appended claims.

1. A lighter-than-air aircraft comprising: a gas envelope for containinga buoyant gas; a support structure within said gas envelope for movingsaid gas envelope from a retracted position to an expanded position,said support structure comprising a hoop-truss member, a control memberpositioned above and coupled to said hoop-truss member, and a firstplurality of tie-downs between said control member and a portion of saidgas envelope positioned above said control member, with a length of saidfirst plurality of tie-downs being increased as said gas envelope movesfrom the retracted position to the expanded position; at least one solarpanel carried by a predetermined portion of said gas envelope; at leastone solar sensor for determining a direction of the sun; and apropulsion system carried by said gas envelope for orienting said gasenvelope so that said at least one solar panel is oriented in thedirection of the sun based upon said at least one solar sensor tothereby increase solar energy collection efficiency.
 2. Alighter-than-air aircraft according to claim 1 further comprising anavigation controller cooperating with said propulsion system to movethe lighter-than-air aircraft along a desired flight path while said atleast one solar panel is oriented in the direction of the sun.
 3. Alighter-than-air aircraft according to claim 1 wherein said gas envelopeis substantially symmetrical about a vertical axis and comprises anupper portion having a partial spheroidal shape; and wherein said atleast one solar panel is carried by a predetermined segment of saidpartial spheroid.
 4. A lighter-than-air aircraft according to claim 1wherein said propulsion system comprises an electrical propulsionsystem; and further comprising at least one closed loop fuel cellcarried by said gas envelope for powering said electrical propulsionsystem when said at least one solar panel is not generating sufficientelectricity, and having its fuel regenerated by said at least one solarpanel from its exhaust when said at least one solar panel is generatingsufficient electricity.
 5. A lighter-than-air aircraft according toclaim 1 further comprising a gondola carried by said gas envelope; andwherein said support structure further comprises a second plurality oftie downs between said control member and said gondola, with a length ofsaid second plurality of tie-downs being increased as said gas envelopemoves from the retracted position to the expanded position.
 6. Alighter-than-air aircraft according to claim 5 further comprising: atleast one electric motor controlling the length of said first and secondplurality of tiedowns; and an envelope controller connected to said atleast one electric motor for control thereof based upon an altitude ofthe lighter-than-air aircraft.
 7. A lighter-than-air aircraft accordingto claim 1 further comprising a gondola carried by said gas envelope;and wherein said propulsion system comprises: a plurality of spacedapart propellers extending outwardly from said gondola; and a respectiveelectric motor for driving each of said propellers.
 8. Alighter-than-air aircraft according to claim 7 wherein said propulsionsystem further comprises a respective gimbal coupled to each of saidpropellers.
 9. A lighter-than-air aircraft according to claim 1 whereinthe lighter-than-air aircraft is unmanned.
 10. A lighter-than-airaircraft comprising: a gas envelope for containing a buoyant gas andbeing symmetrical about a vertical axis; a support structure within saidgas envelope for moving said gas envelope from a retracted position toan expanded position, said support structure comprising a hoop-trussmember, a control member positioned above and coupled to said hoop-trussmember, and a first plurality of tie-downs between said control memberand a portion of said gas envelope positioned above said control member,with a length of said first plurality of tie-downs being increased assaid gas envelope moves from the retracted position to the expandedposition; at least one solar panel carried by a predetermined portion ofsaid gas envelope; at least one solar sensor for determining a directionof the sun; and a propulsion system carried by said gas envelope fororienting said gas envelope so that said at least one solar panel isoriented in the direction of the sun based upon said at least one solarsensor to thereby increase solar energy collection efficiency.
 11. Alighter-than-air aircraft according to claim 10 further comprising anavigation controller cooperating with said propulsion system to movethe lighter-than-air aircraft along a desired flight path while said atleast one solar panel is oriented in the direction of the sun.
 12. Alighter-than-air aircraft according to claim 10 wherein said gasenvelope comprises an upper portion having a partial spheroidal shape;and wherein said at least one solar panel is carried by a predeterminedangular segment of said partial spheroid.
 13. A lighter-than-airaircraft according to claim 10 wherein said propulsion system comprisesan electrical propulsion system; and further comprising at least oneclosed loop fuel cell carried by said support structure for poweringsaid electrical propulsion system when said at least one solar panel isnot generating sufficient electricity, and having its fuel regeneratedby said at least one solar panel from its exhaust when said at least onesolar panel is generating sufficient electricity.
 14. A lighter-than-airaircraft according to claim 10 further comprising a gondola carried bysaid gas envelope; and wherein said support structure further comprisesa second plurality of tie downs between said control member and saidgondola, with a length of said second plurality of tie-downs beingincreased as said gas envelope moves from the retracted position to theexpanded position.
 15. A lighter-than-air aircraft according to claim 14further comprising: at least one electric motor controlling the lengthof said first and second plurality of tie-downs; and an envelopecontroller connected to said at least one electric motor for controlthereof based upon the altitude of the lighter-than-air aircraft.
 16. Alighter-than-air aircraft according to claim 10 further comprising agondola carried by gas envelope; and wherein said propulsion systemcomprises: a plurality of spaced apart propellers extending outwardlyfrom said gondola; and a respective electric motor for driving each ofsaid propellers.
 17. A lighter-than-air aircraft according to claim 16wherein said propulsion system further comprises a respective gimbalcoupled to each of said propellers.